The development of wireless communication systems has increased the demand for monolithically integrated, low-cost and low-phase-noise voltage controlled oscillators (VCOs). At the same time, the development for several communication standards utilizing different frequency bands has pushed researchers to develop multi-band as well as multi-stand transceivers. This has, in turn, pushed researchers to look for multi-band VCOs, especially dual-band VCOs. One of the major demands of such transceivers is the need for multi-band VCOs with good phase noise performance.
Known multi-band VCOs have been realized according to a variety of approaches. One known approach is to use switched capacitors or inductors in a LC resonator, as shown in FIG. 1a-b [1], [2]. The inductance or capacitance of the resonator is thus varied by utilizing switchers Vswitch, consequently also the oscillation frequency-band is varied. Alternatively, the total inductance of the resonator can be varied by switching mutual inductance within a resonator, as shown in FIG. 2 and FIG. 3, [3], [4].
All the above described known solutions suffer from a common problem. The parasitic resistance of the switcher is it via actual switchers or switched mutual inductance degrades the quality factor Q of the resonator. Consequently, the phase noise performance of the entire VCO is degraded. Even though increasing the physical size of the switcher can reduce the parasitic resistance to a certain extent, the large parasitic capacitance results in a decrease of the tuning frequency range.
Yet another known approach has been to build two different frequency VCOs in combination, and let the two VCO's share part of the resonator. By switching the bias current, one VCO works and the other stands by according to [5]. For example, as shown in FIG. 4, there are two VCOs. The resonator of VCO1 consists of one port of a transformer and capacitors, while the resonator of VCO2 consists of two ports of the transformer and capacitors. Thus, two voltage controlled oscillators VCO1, VCO2 have different oscillation frequencies, and they are controlled by a switched current bias. Another example is shown in FIG. 5 [6], where either of the two voltage controlled oscillators VCO1, VCO2 can be turned on by switching the respective bias current Id and Ic. Those two oscillators oscillate at a respective frequency, thus a dual band VCO is obtained.
For the above described switched bias current, only one VCO works at any one time, but the inactive or stand-by VCO is still connected to the resonator, consequently, the parasitic resistance and capacitance of the active devices in the switched-off VCO has certain undesired effects on the operating VCOs.
Due the above problems, there is a need for an improved dual band voltage controlled oscillator with reduced phase noise performance.